Quadrant is the fourth part of a circle and subtends an angle of 90° at the centre. The word has also been applied to an astronomical instrument for measuring the zenith distances of stars, etc. It is placed in the plane of the meridian and generally attached to a wall; hence it is often known as a mural quadrant. A telescope movable about the centre of the quadrant is provided with a vernier, and moves over the divided arc whose zero is the end of a vertical radius. Tycho Brahe used a quadrant also for measuring azimuths, and in this case it was adjusted on a vertical axis. In modern times the quadrant has been almost entirely superseded by the mural circle (q.v.), in which much greater accuracy can be obtained, even though the instrument itself be smaller. In the quadrant error, due to the centre of revolution not coinciding with the centre of the division, is certain to occur; but in the mural circle this is at once eliminated by taking readings on two opposite verniers. Hadley's quadrant (q.v.) is now usually known as Hadley's sextant.